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ARTICLE pubs.acs.org/crystal

Fully Dense Yttrium-Substituted Hydroxyapatite Coatings with Aligned Crystal Domains
Xue Wei, Cong Fu, Keith Savino, and Matthew Z. Yates*
De

partment of Chemical Engineering and Laboratory for Laser Energetics, 206 Gavett Hall, University of Rochester, Rochester, New York 14627, United States ABSTRACT: Continuous yttrium-substituted hydroxyapatite (Y-HAP) coatings on titanium substrates were produced by an electrochemical/hydrothermal method. Hydroxyapatite seeds were ?rst deposited on the titanium surface by electrochemical deposition. Subsequent hydrothermal crystal growth of Y-HAP onto the seed layer was investigated over a range of experimental conditions. Under certain conditions, it was discovered that well-crystallized and fully dense Y-HAP ?lms were produced consisting of crystal domains oriented with the crystallographic c-axis normal to the substrate. At elevated temperatures, protons conduct along the c-axis of Y-HAP crystals. The novel ?lm microstructure can therefore potentially be exploited in new applications of Y-HAP in bioelectrets, fuel cells, and electrochemical sensors.

’ INTRODUCTION Hydroxyapatite (Ca10(PO4)6(OH)2 or HAP) is similar in composition to naturally occurring apatites in teeth and bones, and is often coated onto biomedical implants to improve biocompatibility.1 A variety of methods have been developed to coat HAP onto metal substrates, including sol?gel synthesis, electrochemical deposition, sputter coating, hydrothermal deposition, and electrophoretic deposition.2?8 The most common substrates coated with HAP are titanium and stainless steel because they are used in load bearing orthopedic implants. The bioactivity of HAP strongly depends on mechanical and dissolution properties of coated ?lms. Amorphous or poorly crystalline HAP ?lms dissolve easily in vivo, or have poor adherence to the substrate.6,7,9 Continuous and highly crystallized HAP ?lms with good substrate adherence are preferable for improved durability and biocompatibility. The crystallinity and crystal orientation of HAP can also signi?cantly a?ect electrical properties of the coatings. High temperature electrochemical investigations reveal HAP to be a purely proton conducting material, with proton conductivity of 10?9?10?7 S/cm from 500 ?800 °C.10?12 The mechanism of proton conductivity is thought to involve the shuttling of protons along hydroxyl groups that line the crystallographic c-axis.13,14 At elevated temperatures, an applied electric ?eld will cause HAP crystals to become polarized along the c-axis due to the ?eldinduced movement of protons.14 If the electric ?eld is maintained as crystals are cooled, then the crystals will have permanent electrical polarization along the c-axis at room temperature. Numerous studies have shown that bone growth is improved on the negatively charged hydroxyapatite crystal surfaces of polarized crystals.15?18
r 2011 American Chemical Society

The electrochemical properties of HAP suggest that it may also be e?ective as a proton-conducting electrolyte for fuel cells and other electrochemical devices. Measurements of high temperature electrical polarization and depolarization of HAP reveal an onset of proton mobility within the crystals at temperatures as low as 200?250 °C.14 While protons are quite mobile along the c-axis of individual HAP crystals, the proton conductivity of bulk sintered HAP ceramics is relatively low.10 There is evidence that boundaries between crystal domains act as traps for protons, lowering the overall conductivity.10 We recently reported a novel approach to improve proton transport through HAP membranes by microstructural engineering.19 The concept is analogous to the microstructural engineering of molecular sieve membranes to improve mass transport.20 Our synthetic approach produced hydroxyapatite membranes consisting of aligned crystal domains with the c-axes spanning the membrane thickness. Thus, the ion transport path length is minimized and grain boundary resistance to ion transport is eliminated. The membranes with structure optimized for proton transport displayed proton conductivity nearly 4 orders of magnitude higher than traditional sintered ceramics of the same material.19 The control of the coating’s microstructure was achieved through multiple hydrothermal crystallization steps with surfactants added to alter the relative growth rates of the crystal c-axis versus a-axis. The seeded hydrothermal growth was found to produce dense, well-crystallized coatings on metal substrates.21 In the present study, we investigate the microstructural engineering of yttrium-doped hydroxyapatite (Y-HAP) coatings
Received: July 21, 2011 Revised: November 2, 2011 Published: November 10, 2011
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Crystal Growth & Design on titanium. Yttrium doping has been shown to enhance osteoblast adhesion and calcium deposition onto hydroxyapatite coatings, indicating that Y-HAP may be preferable to pure HAP in improving the performance of orthopedic and dental implants.22,23 The Y3+ ions substitute for Ca2+ in the crystal framework, causing the number of hydroxyl ions to decrease since OH? is transformed to O2? for electrical charge compensation. As a result, yttrium doping also signi?cantly a?ects ion conduction at elevated temperatures. The chemical formula of Y-HAP is given by the following: Ca10?xYx(PO4)6(OH)2?xOx where “x” is the composition parameter for yttrium.24 As x is increased from 0 to 0.65, the proton conductivity of bulk Y-HAP rises from ?10?7 to ?10?4 S/cm at 800 °C, while the activation energy for conduction falls from ?1.5 to ?1.0 eV.25 At higher levels of yttrium doping, Y-HAP transitions from a pure proton conductor at x = 0.65 to a pure oxygen ion conductor at x > 1.25,26 The transition to an oxygen ion conductor is accompanied by a fall in conductivity and rise in activation energy for ion conduction. We recently reported the hydrothermal crystal growth of Y-HAP onto palladium substrates seeded with HAP nanocrystals.27 During hydrothermal crystal growth, yttrium doping was found to provide the dual bene?t of improving conductivity and promoting the formation of fully dense ?lms with near perfect alignment of the crystallographic c-axis normal to the substrate. The resulting microstructure promotes proton transport through the membrane. Y-HAP ?lms on palladium with the composition parameter x ≈ 0.65 displayed the remarkably high proton conductivity of nearly 10?2 S/cm at 700 °C, which is comparable to many ceramic electrolytes used in fuel cells and other electrochemical devices.27 Here, we report a more detailed study of hydrothermal crystal growth of Y-HAP onto titanium substrates to obtain fully dense coatings with the c-axis oriented normal to the substrate. The microstructure is ideal for electrically polarizing the crystals in the ?lm with the dipole oriented normal to the surface. The novel Y-HAP coatings may display improved performance in dental and orthopedic implants, as well as in electrochemical devices.

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Figure 1. SEM images of HAP seeds on Ti substrate prepared by electrochemical deposition at 12.5 mA/cm2 current for 5 min: (a) topview and (b) side-view. The seeded substrate was placed in a Te?on-lined vessel with the seed layer facing down and tilted up at about 45° relative to the bottom of the vessel. For a typical hydrothermal solution, Na2EDTA (0.115 M) was ?rst completely dissolved into 30 mL deionized water. Ca(NO3)2 (0.1 M), Y(NO3)3, (0.01 M), and (NH4)2HPO4 (0.06 M) were successively added and the solution was stirred for 30 min. The salts, particularly nitrates, are very hygroscopic and should not be exposed to humid air. The solution was adjusted to pH = 10.0 with ammonium hydroxide and then transferred to the Te?on-lined vessel to immerse the seeded substrate. The Te?on-lined vessel was sealed in a stainless steel autoclave and put into a convective oven for hydrothermal synthesis at 200 °C for 15 h. After the reaction, the autoclave was cooled to room temperature in air. The sample was taken out, rinsed with deionized water several times, and dried in air. Morphology of the products was examined using a ?eld emission source scanning electron microscope (FESEM, Zeiss-Leo DSM982). Some SEM images have brighter regions due to electrical charging because the samples were not sputter coated with metal prior to SEM analysis. Coating thickness was estimated by side view SEM images of fractured ?lms. The reported thickness was an average of several measurements. ImageJ software was used to calculate the fraction of the top-view SEM image area occupied by crystals. An estimate of ?lm porosity was obtained by assuming that the fraction of the ?lm occupied by crystals is constant throughout the ?lm volume. The crystal structure was determined by X-ray di?raction (XRD) (Philips PW3020) with Cu Kα radiation (λ=1.5418 ?) in the 2θ range from 20° to 40°. The crystallographic texture of HAP seeds and Y-HAP dense ?lm was evaluated by pole ?gures of the (002) plane using a Philips Hi-Resolution X’PERT PRO X-ray di?ractometer (PANalytical, Netherlands). The (002) peak intensity was measured at every 5 degrees of tilt angle, starting from 0° (normal to the substrate) to 85°. At each tilt angle, data were collected as the sample was rotated 360° azimuthally. In each plot, the intensity was normalized to the most intense peak. The intensity plots represent the distribution of (002) planes relative to the substrate surface. The strength of adhesion of the fully dense coating was qualitatively measured by the ASTM standard D3359 tape test.

’ EXPERIMENTAL SECTION
Materials. K2HPO4 (99.99% purity), CaCl2 2H2O (99+% purity), NaCl (99.0% purity), tris(hydroxymethyl)-amino-methane (99.8+% purity), Ca(NO3)2 3 4H2O (99.0% purity), and disodium ethylenediaminetetraacetate dihydrate (Na2EDTA 3 2H2O) (99.0?101.0% purity) were all obtained from Sigma-Aldrich. (NH4)2HPO4 (>99.0% purity) was purchased EMD. 37% hydrochloric acid and 28.0?30.0% ammonium hydroxide were purchased from Mallinckrodt Chemicals. Y(NO3)3 3 6H2O (99.9% purity), titanium (Ti) foil (0.89-mm thick), and platinum (Pt) foil (0.127-mm thick) were obtained from Alfa Aesar. Sample Preparation and Characterization. HAP seeds were deposited onto the Ti substrate through the electrochemical deposition method.4,19,28 In brief, a precleaned Ti plate (12.5 ? 12.5 ? 0.89 mm) was used as the cathode and the anode was a platinum plate (25 ? 25 ? 0.127 mm). The electrolyte solution consisted of 138 mM NaCl, 50 mM tris(hydroxymethyl)-aminomethane, 1.3 mM CaCl2, 0.84 mM K2HPO4 in deionized water. The solution was buffered to pH 7.2 using 37% hydrochloric acid. The electrochemical reaction was carried out at ?95 °C for 5 min with constant current density of 12.5 mA/cm2. After the deposition, the cathode electrode seeded with HAP crystals was taken out of the electrolyte solution, rinsed with deionized water several times, and dried in air.

’ RESULTS AND DISCUSSION Seed crystal layers were deposited onto titanium through electrochemical deposition under conditions similar to those previously reported in the literature.4,19,28 In the electrochemical process, a direct current is applied through the electrolyte solution containing dissolved calcium and phosphate. Hydroxyapatite nucleates on the surface of the metal cathode due to the accumulation of calcium ions by electrostatic attraction and the local increase in pH due to the electrolysis of water. Over long deposition times, hydrogen gas evolution on the cathode surface tends to dislodge deposited crystals, making coatings of uneven
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Figure 2. Side-view SEM images of Y-HAP coatings as a function of reaction time in the hydrothermal process: (a) 5 h, (b) 15 h, and (c) 30 h.

thickness. To overcome this limitation of electrochemical deposition, the reaction was carried out for only a short period of time to ensure a uniform coating. Figure 1 shows the top-view and side-view of the HAP seed layer formed on the titanium substrate in 5 min at 95 °C with a constant current density of 12.5 mA/cm2. The crystals in the seed layer typically have a needlelike morphology of ?1 to 2 μm in length and submicrometer width. The crystal morphology is consistent with previous reports of electrochemical deposition, and the longest axis of the rod-shaped crystals is associated with the crystallographic c-axis. The deposited HAP seeds provided nucleation sites for subsequent hydrothermal crystal growth of Y-HAP into a dense crystalline ?lm on the substrate. For hydrothermal crystallization, EDTA was added to the synthesis mixture in order to chelate the calcium and yttrium ions. The growth was carried out at 200 °C and pH = 10 with an aqueous solution of Ca-EDTA complex, Y-EDTA complex, ammonium phosphate and ammonium hydroxide to adjust solution pH. As the reaction mixture is heated, the EDTA complexes are broken down to release free calcium and yttrium cations. The free cations then react with ammonium phosphate to nucleate and grow Y-HAP. EDTA thus regulates supersaturation of Y-HAP in order to promote crystal growth onto the seeded surface and limit homogeneous nucleation in solution.19 Figure 2 shows SEM images of Y-HAP hydrothermally grown on seeded titanium substrates. Figure 2(a) shows that the Y-HAP crystals grown for 5 h have a size of ?1 μm in width and ?7 μm in length. As the reaction time was increased to 15 h, the width of the crystals increased to ?5 μm and the length was increased to ?11 μm, as shown in Figure 2(b). Crystals grown for 30 h are similar in size to those prepared for 15 h, as shown in Figure 2(c). Extending the growth time to 45 h also produced crystals similar in size (not shown) to those produced after 15 h. The hydrothermal crystal growth onto the seeded substrate is initially rapid, but essentially stops beyond 15 h. In the beginning of the reaction, su?cient amount of reactants are available so that crystals grow longer and wider with time. As the
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Figure 3. Side-view SEM images of hydrothermally crystallized Y-HAP coatings on titanium as a function of the initial pH: (a) pH = 8, (b) pH = 10, and (c) pH = 11.

Figure 4. Side-view SEM images of Y-HAP coatings on titanium as a function of starting [Ca] in the hydrothermal process: (a) [Ca] = 0.05 M, (b) [Ca] = 0.1 M, and (c) [Ca] = 0.4 M.

reaction proceeds, crystal growth slows as reactants are depleted due to growth on the seeded substrate as well as homogeneous crystal nucleation from solution. On the basis of the results in Figure 2, a hydrothermal growth time of 15 h was chosen as a standard synthesis condition. Figure 3 shows the e?ect of varying solution pH on the morphology of Y-HAP crystals grown on the seeded substrate for 15 h at 200 °C. It was found that the thickness and density of
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Figure 5. Side-view SEM images of Y-HAP coatings as a function of starting [P] in the hydrothermal process: (a) [P] = 0.03 M, (b) [P] = 0.09 M.

the Y-HAP coating both increased with increasing pH. Figure 3(a) shows that the Y-HAP crystals in the coating have a size of ?1?2 μm in width and ?10 μm in length when pH was 8. As pH was increased to 10 and 11, the width of the crystals increased to ?5?7 μm, while the coating thickness increased to ?11 μm. The release of free calcium and yttrium from EDTA complexes is regulated by pH as well as temperature. As pH is increased, the dissolution of the EDTA complexes is slowed, which leads to less free calcium and yttrium ions.8 In addition, the solubility of most apatites also decreases with increasing pH.1,29 A more dense coating on the substrate is obtained as pH is increased, but a complete understanding of the factors responsible for morphological changes in the ?lm would require determination of speciation of all compounds under the experimental conditions. The in?uence of calcium concentration on the Y-HAP crystal morphology produced at 200 °C for 15 h at pH = 10 is shown in Figure 4. The starting calcium concentration was varied from 0.05, 0.1, and 0.4 M, while the yttrium concentration was ?xed at 0.01 M and phosphate concentration was ?xed at 0.06 M. To fully complex the calcium added, the amount of EDTA was adjusted according to the equation: [EDTA] = [Ca] + 1.5[Y]. The crosssectional SEM images reveal that the thickness of the coatings decreases slightly while the density increases signi?cantly as calcium concentration is raised. When the starting calcium concentration is 0.05 M, the coating thickness is around 12 μm and many gaps between crystals are observed to extend down to the bottom of the coating. As the calcium concentration was increased to 0.4 M, most crystal domains grew together to form a dense ?lm with a thickness of 10 μm. The side view image in Figure 4(c) shows the gaps between crystal domains on the upper surface do not extend throughout the ?lm. Studies of hydroxyapatite as a liquid chromatography packing have shown that the crystal facets parallel to the c-plane (or c-surfaces) are negatively charged while those parallel to the (a, b) plane (or a-surfaces) are positively charged.30,31 Since a small fraction of Y substitution does not a?ect the structure of HAP crystals due to the similar size of Y3+ and Ca2+, we postulate the surface charges of Y-HAP facets are similar to that of HAP. It is expected that the positively charged calcium ions are preferentially absorbed to the negative charged c-surfaces. When a nonreactive molecule or ion preferentially adsorbs onto one facet of a growing crystal, it lowers the interfacial free energy of that facet, and the crystal growth onto that facet is slowed relative to others.32,33 However, calcium also participates in the crystal growth reaction so that increasing calcium concentration can promote lateral intergrowth of crystals along the a-axis to
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Figure 6. Side-view SEM images of Y-HAP coatings as a function of starting [Y] in the hydrothermal process: (a) No Y, (b) [Y] = 0.0067 M, (c) [Y] = 0.02 M, and (d) [Y] = 0.05 M.

produce a dense coating as a result of an increase in local supersaturation. The e?ect of phosphate concentration on the Y-HAP coatings was investigated by varying the initial phosphate concentration from 0.03 and 0.09 M, while the concentration of calcium was held constant at 0.1 M and yttrium held constant at 0.01 M. As shown in Figure 5, increasing phosphate concentration results in thicker ?lms consisting of larger crystals. With 0.03 M phosphate, the crystal width and length in the coatings were around 1 and 5 μm, respectively. Increasing in the phosphate concentration to 0.09 M produced crystals ?6 μm in width, while the length was increased to ?15 μm. It is expected that the negatively charged phosphate ions preferentially adsorb onto the positively charged a-surfaces. As a result, high phosphate concentration tends to form crystals with longer c-axis. Since the c-axis is preferentially oriented normal to the surface, the thickness of the ?lms is increased. A series of experiments were carried out with varying initial yttrium concentration while holding calcium concentration ?xed at 0.1 M, and phosphate concentration ?xed at 0.06 M. Again, the EDTA concentration was adjusted to ensure complete complexation of the cations according to the formula: [EDTA] = [Ca] + 1.5[Y]. A control experiment without the yttrium salt was also performed. After hydrothermal crystal growth for 15 h at 200 °C, homogeneous coatings of relatively uniform thickness were obtained for all samples. Without yttrium addition, HAP crystals in the coatings are rod-like in shape with a length of ?15 μm as shown in Figure 6(a). Added yttrium promotes lateral intergrowth of crystals for creating a dense ?lm. For example, the width of crystals without the yttrium addition is less than 2 μm, while those with 0.067 M yttrium are up to 6 μm, as observed in Figure 6(a),(b). This is possibly due to the strong adsorption a?nity of yttrium ions to c-surfaces promoting growth of wider crystals. A study of hydroxyapatite doped with various ions has shown that Y-HAP can absorb a greater amount of calcium compared with both undoped and HAP substituted with other
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Figure 7. XRD patterns of HAP and Y-HAP coatings on titanium substrates: (a) HAP seeds, (b) HAP ?lm without Y, (c) Y-HAP ?lm with [Y] = 0.0067 M, (d) Y-HAP ?lm with [Y] = 0.01 M, (e) Y-HAP ?lm with [Y] = 0.02 M, and (f) Y-HAP ?lm with [Y] = 0.05 M.

Figure 8. SEM images of fully dense Y-HAP coating on titanium prepared by electrochemical-hydrothermal method: (a) top-view image and (b) side-view image.

ions.22 When starting yttrium content is less than 0.02 M, the obtained crystals are aligned and oriented with the c-axis normal to the substrate with the c-axis length greater than 10 μm, as shown in side view images in Figures 4(b) and 6(b). However, as [Y] was increased to 0.02 M, a remarkable morphology change in the coatings was observed. The ?lms are ?2 μm thick, containing interconnected crystals with a well-de?ned hexagonal habit, but the c-axes of the hexagonal crystals are no longer vertically aligned, as shown in Figure 6(c),(d). For all samples shown in Figure 6, the initial pH was 10 and after the hydrothermal reaction it was ?9.7. The morphological changes in the Y-HAP coatings may result from the di?erent positions of Y in the crystal framework when yttrium concentration is varied. There are two nonequivalent Ca sites available for cation substitution in an apatite unit cell. Ca(I) is positioned on the 3-fold axis and is surrounded by nine phosphate oxygen atoms, and Ca(II) is at the apex of staggered equilateral triangles surrounded by seven oxygen atoms.26,31,34 It has been reported that at low doping levels the Y cations are randomly distributed among these two type sites, while at the high substitution levels, they go preferentially to the Ca(II) site.26 The location of yttrium in the crystal framework will in?uence the growth rate of di?erent facets to regulate the crystal morphology. X-ray di?raction con?rms the morphological changes observed by SEM, as shown in Figure 7. Figure 7(a) shows the di?raction pattern of the seed layer. The di?raction peaks from the seed layer are consistent with HAP, and no secondary phases were observed. The ratio of the intensities of the (002) to the (300) plane is quite large compared to the standard ratio of the random hydroxyapatite, which indicates a (002) (i.e., c-axis) preferred orientation normal to the substrate. Some HAP crystals are also oriented in other directions since several other di?raction peaks are still observed in Figure 7(a). This is in agreement with the cross-sectional observation as shown in Figure 1(b). After hydrothermal growth, the di?raction peaks are obviously sharper and stronger compared to the crystals in the seed layer, which suggests a higher degree of crystallization. The most notable feature of the pattern after hydrothermal growth is the enhancement in the intensity of the peak at 2θ of 25.8° which corresponds to the (002) re?ection. The enhanced (002) intensity indicates that the c-axes of crystals are dominantly oriented normal to the substrate, consistent with the SEM observation in Figures 4(b), 6(a) and 6(b). The di?raction pattern in Figure 7(b) is consistent with the preferred crystal orientation of the sample
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Figure 9. XRD pattern of fully dense Y-HAP coating on titanium prepared by electrochemical-hydrothermal method.

observed in Figure 6(a). When the starting [Y] is less than 0.02 M, the substitution of Y for Ca does not change the hexagonal crystal structure and no impurity is present, according to the XRD patterns of Figure 7(c),(d). However, as the Y concentration increased to 0.02 M, an enhancement in intensity of the (211) di?raction peak is observed in the pattern, indicating the crystals grow along the (211) direction normal to the substrate. This is also in agreement with the morphological changes as observed in Figure 6. The Na2Y2(PO4)2 phase appears in addition to the apatite phase when Y content is 0.02 M or greater, indicating that some excess Y3+ did not substitute in the apatite structure and reacted directly with PO43?. The e?ects of reagent concentrations, reaction time, and pH observed above were used as a guide to select conditions for growth of dense Y-HAP ?lms. High calcium concentration and moderate yttrium concentration promotes lateral intergrowth of crystal domains, while high phosphate concentration promotes thicker ?lms. Since growth kinetics slows after 15 h, the hydrothermal growth step was repeated using a synthetic solution containing 0.3 M Ca(NO3)2, 0.015 M Y(NO3)3, and 0.18 M (NH4)2HPO4. Each growth step was carried out for 15 h at 200 °C with the starting pH of 10. After each step, the sample was taken out, rinsed with deionzed water and then placed into a new reactant solution to resume the hydrothermal reaction. The obtained Y-HAP coating appears as a uniform and strongly adherent layer on the substrate. The porosity of coatings decreased dramatically with repeated hydrothermal crystallization steps. After repeating the hydrothermal step three times, all crystal domains grow together to form a fully dense ?lm with a thickness approximately 40 μm as shown in Figure 8. Analysis of
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Figure 10. Pole ?gures of (002) peak of (a) seed layer shown in Figure 1 and (b) fully dense Y-HAP ?lm shown in Figure 8. The tilt angle and rotation angle in degrees is plotted in Cartesian coordinates in the x?y plane, while relative intensity of the (002) peak is plotted on the z-axis.

SEM images shows that the ?lm porosity is approximately 20% after the ?rst hydrothermal growth step, and porosity decreases to 0.4% after the third growth step. It should be noted that the porosity is likely overestimated because it was calculated from the fractional area occupied by crystals from the top-view SEM images. Side view images of fractured ?lms show the porosity is lower at the base of the ?lm in contact with titanium. The elemental composition of the dense ?lm was probed with an energy dispersive X-ray spectrometer (Phoenix EDX) attached to the electron microscope. The EDX spectrum is composed of O, P, Ca, and Y peaks, con?rming the presence of Y-HAP. Analysis of the EDX spectrum shows that the Ca/Y atomic ratio is around 12 in the ?lm. The yttrium concentration is enhanced in the Y-HAP ?lm relative to the starting synthesis solution that had a Ca/Y ratio of 20. The X-ray di?raction pattern of this ?lm is shown in Figure 9. The only visible di?raction peak is from the (002) planes, indicating that all crystal domains are near perfectly aligned with the c-axis normal to the substrate. Figure 10 shows three-dimensional pole ?gures of the (002) peak from the fully dense Y-HAP ?lm shown in Figure 8 compared to that of the seed layer shown in Figure 1. The x?y plane in Figure 10 is in Cartesian coordinates obtained by converting the polar coordinates of tilt angle and rotation angle in degrees. For a sample with perfect alignment of the c-axis normal to the substrate, the pole ?gure would appear as a point of maximum intensity at 0°, and falling to zero intensity as the sample is titled. The ?gure shows that the seed layer has preferential orientation of the c-axis normal to the substrate as the maximum intensity is centered around 0° of tilt angle. However, signi?cant intensity from the (002) planes of the seed layer are measured at all tilt angles, indicating some fraction of the seed crystals are oriented normal to all tilt angles measured. In contrast, the fully dense ?lm displays near perfect alignment of the c-axes normal to the substrate. For the fully dense Y-HAP coating, the intensity falls to zero for all tilt angles greater than ?20°. The results indicate that all crystal domains in the fully dense coating are oriented within ?20° of normal to the substrate. This is because the crystal growth in repeated reactions maintains the orientation of the original columnar crystals from the ?rst hydrothermal reaction. The preferentially oriented crystal domains substantially span the ?lm thickness so that no misaligned crystals are detected in the di?raction pattern.

The Y-HAP coatings are fairly durable and adherent to the titanium substrate. A fully dense ?lm similar to the one shown in Figure 8 was subjected to the ASTM standard D3359 tape test. This test calls for a cross shape or hatch pattern to be cut into a coating with a sharp blade. Then, an adhesive tape (Elcometer 99) is applied across the coating and removed. The coating’s adhesive strength is characterized qualitatively based on the amount of coating removed by the tape. The brittle nature of Y-HAP resulted in much of the ?lm cracking and ?aking as the cross shape was cut into the ?lm with a razor. Essentially, all of the Y-HAP ?lm was removed in the area cut by the blade before the tape could be applied. However, if the Elcometer 99 tape was applied to the as-synthesized fully dense Y-HAP ?lm, none of the ?lm was lost when the tape was removed. The results show that the Y-HAP coating adheres strongly to the metal, but can be easily damaged by cutting due to brittle fracture. Continuous and durable Y-HAP ?lms on titanium can be potentially useful for load bearing orthopedic implants, particularly considering the enhanced osteoblast adhesion on Y-HAP compared with pure HAP.22 The crystal orientation in the novel Y-HAP coatings is also ideal for electrically polarizing the crystal surfaces to further promote bone growth, and for promoting proton transport through the membrane.27 Unfortunately, the novel Y-HAP ?lms do not have porosity that is desirable in biomedical ceramic implants for promoting in?ltration of surrounding tissue to obtain strong bioadhesion. Future work is needed to determine if the novel Y-HAP ?lm structure reported here will prove advantageous in biomedical implants.

’ CONCLUSIONS Yttrium substituted hydroxyapatite ?lms on titanium were produced by electrochemical seeding followed by hydrothermal crystal growth. The method includes an electrochemical reaction to seed the metal substrate with a thin layer of HAP crystals and then a hydrothermal process to grow the Y-HAP coatings with a dense coverage onto the seeded substrates. The e?ects of several parameters such as reaction time, solution pH, and composition of the reactants were investigated in the hydrothermal process. A solution pH of 10 and hydrothermal reaction time of at least 15 h are favorable for growing dense and well crystallized Y-HAP coatings. Increasing phosphate concentration relative to calcium
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Crystal Growth & Design increases coating thickness, while increasing calcium concentration relative to phosphate promotes dense surface coverage. The e?ect of yttrium concentration is complex, with moderate concentration promoting dense ?lms with (002) orientation normal to the substrate and higher concentration promoting (211) orientation normal to the substrate. Hydrothermal reaction conditions were found that allow complete and uniform coating of Y-HAP onto titanium with near perfect alignment of the c-axes of crystal domains normal to the substrate through repeated hydrothermal crystallization steps. The preferred crystal orientation will facilitate proton transport through the coating thickness, potentially allowing improved performance of polarized ?lms in orthopedic applications and as ion conducting membranes in electrochemical devices.

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’ AUTHOR INFORMATION
Corresponding Author

*Tel: 1-585-273-2335; fax: 1-585-273-1348; e-mail: myates@ che.rochester.edu.

(21) Liu, D.; Savino, K.; Yates, M. Z. Surf. Coat. Technol. 2011, 205, 3975–3986. (22) Webster, T. J.; Ergun, C.; Doremus, R. H.; Bizios, R. J. Biomed. Mater. Res. 2002, 59, 312–317. (23) Sato, M.; Sambito, M. A.; Aslani, A.; Kalkhoran, N. M.; Slamovich, E. B.; Webster, T. J. Biomaterials 2006, 27, 2358–2369. (24) Owada, H.; Yamashita, K.; Umegaki, T.; Kanazawa, T.; Nagai, M. Solid State Ionics 1989, 35, 401–404. (25) Yamashita, K.; Owada, H.; Umegaki, T.; Kanazawa, T.; Katayama, K. Solid State Ionics 1990, 40?1, 918–921. (26) Laghzizil, A.; Bouhaouss, A.; Barboux, P.; Morineau, R.; Livage, J. Solid State Ionics 1993, 67, 137–143. (27) Wei, X.; Yates, M. Z. Chem. Mater. 2011submitted. (28) Ban, S.; Hasegawa, J. Biomaterials 2002, 23, 2965–2972. (29) Zhang, H; Li, S.; Yan, Y. Ceram. Int. 2001, 27, 451–454. (30) Kawasaki, T.; Niikura, M.; Kobayashi, Y. J. Chromatogr. 1990, 515, 125–148. (31) Orlovskii, V. P.; Barinov, S. M. Russ. J. Inorg. Chem. 2001, 46, S129–S149. (32) Berkovitchyellin, Z.; Vanmil, J.; Addadi, L.; Idelson, M.; Lahav, M.; Leiserowitz, L. J. Am. Ceram. Soc. 1985, 107, 3111–3122. (33) Murphy, C. J. Science 2002, 298, 2139–2141. (34) Ergun, C. J. Eur. Ceram. Soc. 2008, 28, 2137–2149.

’ ACKNOWLEDGMENT We acknowledge NSF (CMMI-0856128), the DOE through the Laboratory for Laser Energetics (DE-FC03-92SF19460) and the University of Rochester for supporting this research. ’ REFERENCES
(1) Suchanek, W.; Yoshimura, M. J. Mater. Res. 1998, 13, 94–117. (2) Nelea, V.; Morosanu, C.; Iliescu, M.; Mihailescu, I. N. Appl. Surf. Sci. 2004, 228, 346–356. (3) Yang, Y.; Kim, K.-H.; Ong, J. L. Biomaterials 2005, 26, 327–337. (4) Ban, S.; Maruno, S. J. Biomed. Mater. Res. 1998, 42, 387–395. (5) Cheang, P.; Khor, K. A. J. Mater. Process. Technol. 1995, 48, 429–436. (6) Liu, D.-M.; Yang, Q.; Troczynski, T. Biomaterials 2002, 23, 691–698. (7) Onder, A.; Osman, E.-A.; Sabri, A. Surf. Coat. Technol. 2008, 202, 2482–2487. (8) Haders, D. J.; Burukhin, A.; Huang, Y. Z.; Cockayne, D. J. H.; Riman, R. E. Cryst. Growth Des. 2009, 9, 3412–3422. (9) Xue, W.; Tao, S.; Liu, X.; Zheng, X.; Ding, C. Biomaterials 2004, 25, 415–421. (10) Yamashita, K.; Kitagaki, K.; Umegaki, T. J. Am. Ceram. Soc. 1995, 78, 1191–1197. (11) Gittings, J. P.; Bowen, C. R.; Dent, A. C. E.; Turner, I. G.; Baxter, F. R.; Chaudhury, J. B. Acta Biomater. 2009, 5, 743–754. (12) Tanaka, Y.; Nakamura, M.; Nagai, A.; Toyama, T.; Yamashita, K. Mater. Sci. Eng. B Adv. Funct. Solid State Mater. 2009, 161, 115–119. (13) Maiti, G. C.; Freund, F. J. Chem. Soc.-Dalton Trans. 1981, 949–955. (14) Nakamura, S.; Takeda, H.; Yamashita, K. J. Appl. Phys. 2001, 89, 5386–5392. (15) Nakamura, S.; Kobayashi, T.; Nakamura, M.; Yamashita, K. J. Mater. Sci. - Mater. Med. 2009, 20, 99–103. (16) Itoh, S.; Nakamura, S.; Nakamura, M.; Shinomiya, K.; Yamashita, K. Biomaterials 2006, 27, 5572–5579. (17) Nakamura, S.; Kobayashi, T.; Yamashita, K. J. Biomed. Mater. Res. 2002, 61, 593–599. (18) Baxter, F. R.; Bowen, C. R.; Turner, I. G.; Dent, A. C. E. Annal. Biomed. Eng. 2010, 38, 2079–2092. (19) Liu, D. X.; Savino, K.; Yates, M. Z. Adv. Funct. Mater. 2009, 19, 3941–3947. (20) Lai, Z.; Bonilla, G.; Diaz, I.; Nery, J. G.; Sujaoti, K.; Amat, M. A.; Kokkoli, E.; Terasaki, O.; Thompson, R. W.; Tsapatsis, M.; Vlachos, D. G. Science 2003, 300, 456–460.
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dx.doi.org/10.1021/cg200943s |Cryst. Growth Des. 2012, 12, 217–223


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